Container handling machines are equipment specially used for container loading, unloading, transshipment and stacking operations. In the past, they were driven by wound asynchronous motors, and the speed was adjusted by changing the rotor resistance through a drum cam controller. With the development of power electronics technology and inverter vector control technology, inverters are now widely used to drive asynchronous motors, and PLC control is introduced. As an automatic control device, the inverter has stable and reliable performance. Its application in container loading and unloading systems can significantly improve performance and reliability, and is the current ideal control mode. This improvement optimizes the speed regulation performance and enhances system reliability.This article will elaborate on the variable frequency speed regulation process of FRECON Electric FR500A in container handling machines.
The operating mechanism of the container handling machine is a constant torque load, and its hook drag system is a potential load. When the hook lifts the weight and descends or decelerates quickly, the motor is in a regenerative power generation state. The electric energy needs to be fed back to the grid through the feedback device or consumed in the braking resistor to prevent the inverter bus voltage from overshooting and the inverter from shutting down due to overvoltage protection.
The container loader and unloader mainly consists of three parts: the large vehicle towing system, the trolley towing system and the hook towing system.
(1) The cart dragging system drags the entire crane to move left and right along the direction of the workshop (taking the driver's sitting direction as a reference)
(2) The trolley drag system drags the hook and the weight to move forward and backward along the bridge.
(3) The hook dragging system drags the weight to move up and down to lift or lower. The following introduces the hook dragging system.
The hook dragging system requires a large starting torque, stable operation, forward and reverse operation, and multiple protections such as overload, limit, and current limit.However, the "hook slipping" problem is easy to occur during starting and stopping. The brake takes time to tighten and release (about 0.6s), while the motor torque reacts immediately when the power is on or off. The coordination of the two actions is prone to problems. If the motor is powered on and the brake is not released, it will cause overload; if the motor is powered off and the brake is not tightened, the heavy object will slide down, which is called "hook slipping". Therefore, corresponding preventive measures need to be taken.
The hook dragging system must have a mechanical brake device (mechanical brake). When a heavy object is suspended in the air and there is a sudden power outage, if a mechanical brake device is not installed, the heavy object will be in danger of slipping down the hook. Therefore, a brake must be installed on the hook motor shaft. Commonly used ones include electromagnetic brakes and hydraulic electromagnetic brakes.
The trolley (translation mechanism) drag system of the container loader does not have high requirements on the performance of the inverter. In order to save costs, the V/F control method can meet the requirements. The loader hook (lifting mechanism) drag system requires higher starting torque and speed regulation performance, and a vector control inverter must be used. This article uses the FR500A series vector inverter. The FR500A series inverter has the following characteristics:
The power of the hook lifting motor is: 75kw. In order to ensure sufficient starting and running torque, the capacity of the inverter is enlarged by one specification. The FR500A-4T-090G/110P inverter is selected. In order to save costs and simplify the control system, the PG-free vector control method is adopted.
The braking unit of the inverter should be increased by one level to allow a larger braking current and shorten the braking process; the rated power of the braking resistor should be doubled compared to conventional loads.
The main switch, overload, limit switch and the inverter relay output signal 1 (fault output) are used as the PLC input signal. The PLC output signal controls the inverter's multi-function input terminal (controls the inverter's forward and reverse rotation, multi-speed, fault reset, emergency stop, etc.) and the on and off of the main power circuit.
The touch screen and PLC are connected via an RS422 serial interface. The interface program in the PLC sets up a data reading area and related status signs for the touch screen in the PLC to monitor the height, load, operating status, fault information, etc. of the main hook.
The inverter multi-function input terminals DI1-DI7 are used to control the inverter's start, stop, forward, reverse, multi-speed, fault reset, and emergency stop. Relay 1 output is used as "fault output"; relay 2 is used as "brake/release" (frequency + torque combination) output. The brake unit and brake resistor on the inverter DC bus are used to consume the feedback energy generated during the hook's downward movement.
①Static self-learning
When using vector control without PG, the performance of the inverter control is based on the accuracy of the motor model. Therefore, before running the motor for the first time, it is necessary to self-learn the motor parameters:
②Braking and releasing control
The torque of vector control without PG is insufficient at "0" Hz, so the FDT level function needs to be used to set a suitable detection value so that the inverter can open the mechanical brake after running to a certain frequency. A detection value that is too high may cause "overload" or "overcurrent" faults, and a value that is too low may not be able to lift heavy objects, so it is finally set to 2Hz. In contrast, vector control with PG can directly output a signal to open the brake device at "0" Hz without relying on frequency increase.
For the hook dragging mechanism, the inverter drives a motor, so the inverter output can be directly connected to the motor without connecting the thermal relay for overload protection. The inverter itself has multiple protections such as short circuit, overload, undervoltage, etc. In the main power control circuit, a door limit switch and a key switch are also connected in series as safety circuit protection measures.
With the strong support of inverters and PLC with complete functions and stable and reliable performance, container loaders have greatly improved in reliability, speed regulation performance, energy saving and operating efficiency compared with traditional container loaders. The container loaders system composed of inverters and PLC has become the typical control mode of container loaders and is being used more and more widely.
